How to toggle to camera view via python?
The operator for switching the 3D view which is associated with numpad keys is view3d.viewnumpad().  You have to be in the right context (3d_view) to use it and select which view you want to set:

import bpy

for area in bpy.context.screen.areas:
    if area.type == 'VIEW_3D':
        override = bpy.context.copy()
        override['area'] = area
        bpy.ops.view3d.viewnumpad(override, type = 'CAMERA')
        break


Another way, without the operator would be:

import bpy

for area in bpy.context.screen.areas:
    if area.type == 'VIEW_3D':
        area.spaces[0].region_3d.view_perspective = 'CAMERA'
        break


Which is equivalent to: 

import bpy

area = next(area for area in bpy.context.screen.areas if area.type == 'VIEW_3D')
area.spaces[0].region_3d.view_perspective = 'CAMERA'


Other options are  [‘PERSP’, ‘ORTHO’, ‘CAMERA’]